The invention relates to an arrangement for stroboscopic potential measurements with an electron beam measuring or testing device which exhibits a beam suppression blanking system, and in which the electron beam is deflected across an aperture or diaphragm during each edge of a sampling pulse such that two electron pulses are generated for each sampling pulse.
High frequency signal paths can be stroboscopically measured with an electron beam testing device. Accordingly, a finely focused electron beam which is directed onto the measuring subject, for example an integrated circuit, serves as the test probe. Due to the interaction between electrons and solid bodies, secondary electrons among other things are released which can be employed for imaging an object. These secondary electrons also carry information concerning the electrical potential at the location of incidence. Upon exploitation of the stroboscopic effect, specimens or subjects being tested functioning with a high nominal frequency can also be quasi-statically imaged as a potential contrast. For this purpose, the measuring subject to be examined is targeted with cyclically repeating signals and is imaged in a scanning electron microscope. The electron beam is turned on only once for a brief time in each cycle, i.e. the measuring subject is only observed during a specific phase. Thus, the imaging is a snapshot of the rapidly functioning probe. The point in time at which the electron beam is switched on can be selected at random within the cycle. Slowmotion presentation of the switching operations is possible by means of slowly shifting the phase. The on-time duration of the electron beam can be reduced down to the picosecond range, i.e. the chronological resolution of said imaging method lies in the picosecond range. The electron range are generated with the assistance of a beam suppression or blanking system.
A purpose of the device disclosed herein is to generate the electron pulses and to process the secondary electron pulse signals.
In order to generate short electron pulses, deflection structures (plate capacitors, traveling wave arrangements) which deflect the electron beam from the beam path onto a diaphragm or aperture were previously employed. There are fundamentally two possibilities for the drive of the deflection structures. According to method I (the arrangement specified by H. P. Feuerbaum and J. Otto in J. Phys. E: Sci. Instrum., Vol. 11, 1978, 529-532, incorporated herein by reference, and which can be employed for this purpose), one deflection structure is grounded, whereas a constant voltage is applied to the other deflection structure. A blanking pulse superimposed on the constant voltage places the deflection structure at grounded potential. During the blanking pulse, the space between the two deflection structures is field-free, so that the electron beam is switched on. In this method, the electron pulse width is determined by the width of the blanking pulse applied. The smallest electron pulse width hitherto attainable amounts to 350 ps.
According to method II (K. G. Gopinathan, A. Gopinath, "A Sampling Scanning Electron Microscope", J. Phys. E: Sci. Instrum., Vol. II, 1978, 229-233, incorporated herein by reference, shorter pulse widths (.apprxeq.10 ps) are achieved when one deflection structure is grounded, whereas a negative constant voltage -U.sub.o is applied to the other. A blanking pulse superimposed on the constant voltage places this deflection structure approximately at the potential +U.sub.o. Therefore, the electron beam is deflected across a diaphragm or aperture during each edge of a blanking pulse. In accordance with the rising edge and falling edge, the circuitry supplies two electron pulses per blanking pulse.
Since a stroboscopic measurement, however, requires one electron pulse per signal cycle with which the test subject is driven, this wiring is only suitable when
(a) one of the two electron pulses is deflected out of the beam path by means of an additional beam blanking system; that, however, is only possible at great mechanical and electronic expense. PA1 (b) two respectively successive electron pulses have the same chronological spacing with respect to one another and the frequency of the signal with which the test subject is cyclically driven is twice as high as the frequency of the blanking pulse. In this method, however, a high chronological resolution can be achieved only after a protracted adjustment of the spacing of the two electron pulses.